When a user uses Microsoft Office Communicator Web Access (CWA) to send an IM message to a federated or Public IM Connectivity (PIC) user, CWA may give an incorrect notification of the IM delivery status.
For example, in some scenarios, the message receiver has already received the IM message, but CWA may notify the sender that the message delivery failed. Or, the receiver does not receive the message, but no failure-to-deliver message is returned to the sender.
To resolve this problem, apply the following update:
967836 Description of the update package for Communications Server 2007 R2, Communicator Web Access: April 2009
This update fixes this problem for CWA, and the sender receives the correct delivery status for messages.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.